Design of typical radio-frequency (RF) receivers in cellular mobile terminals are subject to several design constraints. The first constraint is limitations on the ability to reliably detect very weak signals in the desired frequency channel. The second constraint is the ability to detect only slightly stronger in-band signals in the presence of very strong interfering signals. For instance, for the GSM system, the receiver must be able to reliably detect signals with a strength of −108 dBm in the absence of interference and a strength of −99 dBm while in the presence of 0 dBm interfering signals at an offset of 20 MHz or more.
The most common solution to solving problems caused by very strong interfering signals has been to make use of very high quality factor (Q) bandpass filters at the input of the receiver. These filters are typically surface acoustic wave (SAW) filters which pass the receive band with a typical attenuation of ˜2.5 dB and attenuate out-of-band signals (e.g., 10-20 MHz away from the receive band) by about 20 dB. These filters are highly linear and typically result in a reduction of out-of-band interfering signals to about the same level as in band interference (−23 dBm).
There are several drawbacks associated with this approach however. The first is that in-band attenuation tends to make it harder to detect weak signals, creating the need for an even more sensitive receiver after the filter. More importantly, there is currently no economical way to implement SAW filters or their equivalents in the same processes as the active circuits that follow them, which are typically produced using CMOS or BiCMOS processes and either silicon or silicon germanium technologies. The result is that SAW filters significantly increase the cost and consume equally valuable circuit board area in a typical handset. This problem is further exacerbated by the proliferation of different frequency bands that a mobile handset has to be compatible with.
FIG. 1 is a diagram of an exemplary prior art system 100 for providing multiple band compatibility. Since each band has a different pass-band and different stop-bands, each band requires a separate SAW filter 102A, and, consequently separate input ports to the separate receiver inputs 104A as well as separate outputs from any transmit/receive (T/R) switch 106A or similar device.
FIG. 2 is a diagram of a linear-time-varying (LTV) low-pass filter 200 in accordance with the prior art. Filter 200 can be built by combining three capacitors and two switches, such as might be used in the separate and traditionally unrelated area of switched capacitor filters. A differential current of frequency Fsw can be driven across ports Vo+ and Vo−. The bandwidth of the resulting filter is equal to (Ci/Co)·Fsw. Applying a current signal to Vi at a frequency Fsw+dF, where dF is an offset frequency, results in a differential output voltage (Vo+−Vo−) with a frequency dF and a filtered amplitude with bandwidth (Ci/Co)·Fsw. The input voltage Vi is partially filtered by an LTV band-pass filter centered on Fsw with a bandwidth of 2·(Ci/Co)·Fsw.